- asynchronous serial lines, in which the format is self-clocking (i.e. the start location of each character can be inferred from the data stream), and
- synchronous serial lines, where each character starts immediately after the previous one, and is therefore not self-clocking
Both serial line protocols requires two signal levels: idle (mark, often high voltage), and asserted (space, often low voltage). (The polarity is a legacy from telegraphy, where the line was held high in order to show that the line was not broken, and that the transmitter was functional.)
If the line is held in the 'asserted' state for longer than a character time, this is called a break condition, and it can be detected by the receiver and signalled to the user specially. This is often used to interrupt the computer on the other end of the line.